Mesure the distance from the nut to the bridge (where the strings touch the bass a the top of the neck and the bottom of the bass)
This is the scale of the bass, a standard bass is around 34 inches. if the mesurment is significantly shorter you will need short scale strings, longer...

Next think about how tight you want the strings to be. If you want the strings to be realy bendable and kind of loose feeling get a light gauge set of strings.

If you want them to be tight get a heavier set.

you will probalby want a set of standard scale medium-light(45-100) or medium(45-105) gauge strings